Validate that we're properly generating a single line ending on Windows when
using -frewrite-includes. Otherwise we're breaking split-line macros. The test
fails before 23929af383f27a6ddf23704192a25591481152b3.
See discussion in https://reviews.llvm.org/D96363#2650460 and D99426
Differential Revision: https://reviews.llvm.org/D99973
